Your object isn't just thin but doesn't own some thickness at all. It's just a bundle of surfaces.
The stripe pattern means that there's a flipped surface normal direction. There the faces' front points to the inside. This is the first thing to repair. In SELECT's tool options enable AllowBackFaces, paint some selection on each flipped region and hit E to expand the selection to connected faces. Now run Edit/FlipNormals to get the object with the white side pointing outwards all over.
Next is to close open boundaries inside the helmet. Run ANALYSIS/Inspector. Click on the marker spheres which point to those interior cracks one by one. Do not AutoRepairAll which will close all open boundaries.
For the next step it is important that the current object is at the right scale. Run ANALYSIS/UnitsDimensions to set the correct size.
To get a wall thickness select all - simply hit Ctrl+A - and run Edit/Offset. Make sure that Connected is enabled and set Distance to the negative value of the desired wall thickness.
